Michael Tucker, Furmanite International, UK
The vital importance of maintaining a power plant’s major systems such as turbines, generators, boilers and engines goes without saying, but the same attention is not always paid to the maintenance of the balance of plant (BoP).
Schematic of a Tevitest device that can test the performance of safety and relief valves in-situ
Yet the BoP, the term commonly applied to the host of supporting or auxiliary equipment and components connecting the major systems in such a way that the power plant as a whole functions as an effective and efficient entity, is of vital importance in itself.
While the BoP systems such as heat exchangers, pumps, compressors, valves and piping will be different for each power plant type, the common factor is that all plant components must be fully integrated, with each performing to maximum efficiency for optimum economic performance of the plant as a whole in terms of availability and reliability. Monitoring and effective maintenance of the BoP systems is therefore every bit as critical as maintenance of the major systems, and needs to be recognized as such.
The failure of comparatively minor components such as seals and bearings in pumps, valves or pipeline flanges can have significant safety or environmental implications, compromise plant performance, and in the worst case scenario could lead to shutdown and loss of production, at high cost.
Valves are one example. The important role of the safety or relief valve in its various forms within BoP systems has long been recognized. Designed to discharge the required capacity of gas or fluid at a pre-determined pressure to prevent excessive overpressure of pressurized vessels and systems to safeguard life and assets, they will be tested at regular intervals during their design life by the certifying authority. This is done in line with the statutory requirements for fired pressure vessel users to test the functioning of the protective valves at specific intervals, to ensure that they open within a set pressure tolerance to achieve the certified flow rate at a pre-determined pressure, and blow down (reseat) within an allowable pressure drop.
Since numerous factors may impair a safety valve’s performance, with incorrect location or installation, operating conditions, exhaust paths, mechanical condition and poor maintenance among the common causes, testing safety valves is undeniably critical. A key issue is therefore identifying the optimum testing method to keep downtime to a minimum.
Avoiding valve failure
One traditional method is to remove the valve and take it to a workshop where it is pre-popped placed on a test bench and the set pressure checked by increasing pressure underneath the valve until the disc is lifted from the seat then stripped down and evaluated, and parts replaced or machined as necessary to return it to its ‘as new’ condition. The reassembled valve is then returned to the test bench where the set pressure is determined, and the procedure repeated until the correct setting is achieved. A further test ensures that the valve seat is not leaking beyond an acceptable level.
A second option is to prove the set pressure with the valve in-situ, by increasing the system pressure until the valve lifts. The set pressure is recorded, system pressure reduced, and the pressure at which the valve reseats also recorded. If the two sets of figures do not comply within an acceptable tolerance the valve is adjusted and the process repeated until acceptable results are achieved.
A third more recent approach is in-situ testing with a device such as Trevitest. Here the principle is to apply a force to the valve spindle to overcome the spring tension, using a hydraulic power pack attached to a specially designed set of mechanics mounted onto the valve. An electronic force transducer, linked to a portable computer, traces the force applied. The recorded data together with a knowledge of the valve seat area and line pressure enables the set pressure to be determined. Importantly the test is performed with the system operating normally at working pressure. Safety and relief valves can thus be recertified in-situ to ensure that these critical devices are always fit for purpose, and any maintenance work can be aligned with planned major outages.
Joint approach TO Joint integrity
A further instance where failure could have costly consequences lies in joint integrity. Whether pipeline flanges or valve, pump, compressor, pressure vessel or heat exchanger connections, joint integrity may appear insignificant compared to the maintenance of the turbine or other major equipment, but if not properly and effectively managed could become a significant issue. To achieve reliable leak-free joints requires a number of key criteria to be addressed, any of which, if ignored, could result in a leak with a detrimental impact on the BoP system, and therefore on the plant as a whole.
Vital considerations include a review of the flange against the relevant design standard, and identification of the optimum bolt load (incorrect loads, which must be sufficient but not too high, are a common cause of joint failure) and tightening method uncontrolled tightening being another common cause of leaking joints, so bolt tensioning or torquing is required. Further, the gasket design must be evaluated (inappropriate gasket selection is one of the primary causes of leaking flanges), and sealing surfaces checked for surface finish, flatness and condition. Flatness outside the maximum tolerance, or defects greater than 30 per cent of the sealing face width make the joint difficult to seal, while a rougher surface finish needs higher bolt loads to seal effectively.
Flange and bolt materials are a further consideration. Differential thermal expansion, which can cause joints to leak on start-up or coming off-line, but seal when at temperature can be addressed by using alternative bolt materials or altering the bolt grip length, while stress relaxation behaviour of the bolt material can also affect the seal achieved. Flange distortion is another common cause of leaking joints, and any significant misalignment may require additional load to seal effectively.
Additionally, the joint’s physical size, whether it is operating at high temperature or pressure, or at fluctuating temperatures or pressures, or has a history of troublesome performance are all factors to take into account.
Maintenance programmes to manage critical joint integrity must address all these factors and should include a pre-shutdown engineering analysis and documented work requirements, as well as disassembly, inspection, machining, gasket installation, assembly and controlled bolting. Effective and comprehensive record keeping for full traceability and access for future maintenance planning is equally important, preferably maintained electronically with work logged as it is undertaken to provide real-time status, a factor that becomes vital with the latest health and safety requirements for demonstrable up to date maintenance records. The Pressurized Systems Integrity (PSI) Management service from Furmanite provides this, for example.
Reducing downtime and cost
Another focus for BoP maintenance must be heat exchangers, often the weakest link in the efficient running of a plant. A linear flow through the exchanger is critical, with any interruption, such as a reduction in tube diameters through pipe reducers or build-up of deposits, resulting in turbulence, creating erosion and/or corrosion, causing further flow reduction and eventual through-wall defects. Tube plugs can be installed to isolate defective tubes, keeping the exchanger operational and preventing leakage to avoid unscheduled shutdown, but reducing the exchanger efficiency.
Services like PSI Management are designed to manage joint integrity and prevent critical joint failure
Instead, one highly cost-effective route is to install liners (ferules) at the tube entry tube erosion commonly appears in the first 20 cm from the inlet end, after which flow becomes more linear. Installed for any given length either during manufacture or as on-going maintenance these can be made from more exotic or resistant material than the lower grade tube material, increasing durability only where required, avoiding the need to replace entire tubes with only isolated damage. The liners can be replaced during planned shutdowns, thereby extending the life of the original tubes and reducing downtime and associated costs.
hollistic maintenance approach
Addressing these amongst multiple BoP maintenance issues can effectively help close the gap that can be experienced between current equipment performance and corporate objectives, with improvements in plant efficiency and performance, as well as environmental and safety benefits. Maintenance strategies that place emphasis on the major equipment at the expense of the BoP systems could be misplaced.
Effective BoP maintenance can achieve valuable increased availability and maximize uptime, minimize scheduled maintenance outages and improve plant performance. In short, attention paid to maintenance of BoP systems could provide the cumulative benefits and improvements in plant operational efficiency and performance that will make a valuable contribution to bottom line profitability.